Graphene based structures and methods for shielding electromagnetic radiation

ABSTRACT

Electromagnetic interference shielding structures and methods of shielding an object form electromagnetic radiation at frequencies greater than a megahertz generally include providing doped graphene sheets about the object to be shielded. The doped graphene sheets have a dopant concentration that is effective to reflect and/or absorb the electromagnetic radiation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims priority toU.S. Application Ser. No. 13/523,178, filed on Jun. 14, 2012,incorporated herein by reference in its entirety.

BACKGROUND

The present invention generally relates to structures and methods forshielding electromagnetic waves using graphene, and more particularly,to methods and structures of doped graphene sheets configured to reflectand/or absorb the electromagnetic waves being emitted from aelectromagnetic wave generating source.

Emission of electromagnetic (EM) radiation at radio, microwave andterahertz frequencies is known to interfere with operation of electronicdevices and has been linked to various health hazards to exposedindividuals. For example, the World Health Organization has recentlyannounced that exposure to microwave radiation could increase the riskof brain cancer. Because of concerns such as these, EM radiation is aserious issue and attempts to provide various shielding materials anddevices have evolved. Most commonly used EM shields in use today arefabricated from metallic films, metallic grids, metallic foams, orpowders on glass or plastic substrates. One example is a shielded cable,which has electromagnetic shielding in the form of a wire meshsurrounding an inner core conductor. The shielding impedes the escape ofany signal from the core conductor, and also signals from being added tothe core conductor. Some cables have two separate coaxial screens, oneconnected at both ends, the other at one end only, to maximize shieldingof both electromagnetic and electrostatic fields. Another example is thedoor of a microwave oven, which typically has a metallic screen builtinto the window. From the perspective of microwaves (with wavelengths of12 cm) this screen in combination with the oven's metal housing providesa Faraday cage. Visible light, with wavelengths ranging between 400 nmand 700 nm, passes easily between the openings the metallic screenwhereas microwaves are contained within the oven itself.

Due to the inherent weight of metallic shields, the added weight can besignificant. Moreover, many of the currently available EM shields arenot transparent, which can be a significant disadvantage for manyapplications. Conventional transparent and conductive materials such asindium tin oxide (ITO) and zinc oxide (ZnO) have been applied totransparent substrates such as glass and plastics for EM shielding.However, the use of these types of transparent EM shields is fairlylimited in their use because the shielding effectiveness of thesematerials is generally low, the shield itself is typically inflexible,and these types of EM shields provide limited mechanical strength.Providing higher EM effectiveness with these types of materials requiresincreased thicknesses, which then affect transparency.

SUMMARY

According to an embodiment, a method for shielding an object fromelectromagnetic radiation at frequencies greater than a megahertzemitted from an electromagnetic source, comprises providing one or moregraphene sheets on or about the object, wherein at least one or more ofthe graphene sheets are doped with a dopant.

According to an embodiment, a method for shielding an object fromelectromagnetic radiation at frequencies greater than a megahertzemitted from an electromagnetic source comprises providing one or moregraphene sheets on or about the object, wherein at least one or more ofthe graphene sheets are doped with a dopant in amount effective toreflect and/or absorb the electromagnetic radiation.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with advantagesand features, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 illustrates electromagnetic shielding structure for an object tobe shielded from electromagnetic radiation, the structure includingindividually doped graphene sheets according to an embodiment.

FIG. 2 illustrates electromagnetic shielding structure for an object tobe shielded from electromagnetic radiation, the structure including adoped uppermost graphene sheet according to an embodiment.

DETAILED DESCRIPTION

Disclosed herein are electromagnetic shielding structures and methodsfor shielding electromagnetic radiation emitted from an electromagneticradiation source. The electromagnetic shield structures are generallyformed from one or more sheets of doped graphene.

Graphene is atomically thin and has a zero band gap. Its lineardispersion around the K (K′) point leads to constant interbandabsorption (from valence to conduction bands, about 2.3%) of verticalincidence light in a very broadband wavelength range. By doping thegraphene sheets, higher carrier absorption can be obtained as well hashigher transparency in the near infrared and visible wavelength rangesdue to Pauili blocking.

Advantageously, the electromagnetic shield structures according to thepresent invention provide effective shielding by reflection at afrequency range of about 1 megahertz to about a few hundred gigahertz,which is a significant improvement over prior electromagnetic shieldingmaterials. Moreover, because graphene is a one atom thick monolayersheet formed of carbon atoms packed in a honeycomb crystalline lattice,wherein each carbon atom is bonded to three adjacent carbon atoms viasp² bonding, the overall thickness required to provide >40 decibel (dB)shielding effectiveness, for example, is on the order of a fewnanometers. As such, the use of doped graphene sheet(s) provides minimaladded weight to the object to be shielded, has broadband capabilities,and greater versatility as a function of its doping. Moreover, grapheneis generally recognized for its high mechanical strength and highstability. In contrast, prior electromagnetic shield materials requirean increased thickness to increase shielding effectiveness. In thepresent disclosure, increasing the level of doping for a given thicknessof stacked graphite sheets provides increased shield effectiveness.

The graphene sheets can be made by any suitable process known in the artincluding mechanical exfoliation of bulk graphite, for example, chemicaldeposition, growth, or the like. The graphene can be formed on asubstrate as may be desired in some applications. The particularsubstrate is not intended to be limited and may even include theelectromagnetic radiation source itself. Likewise, the shape of thesubstrate is not intended to be limited. For example, the substrate mayhave planar and/.or curvilinear surfaces such as may be found in foils,plates, tubes, and the like. Moreover, the substrate material is notintended to be limited. Suitable materials include plastics, metals, andthe like, which may be rigid or flexible.

By way of example only, chemical vapor deposition (CVD) onto a metal(i.e., foil) substrate can be used to form the graphene sheets. See, forexample, Li et al., “Large-Area Synthesis of High-Quality and UniformGraphene Films on Copper Foils,” Science, 324, pgs. 1312-1314 (2009)(hereinafter “Li”) and Kim et al., “Large-Scale Pattern Growth ofGraphene Films for Stretchable Transparent Electrodes,” Nature, vol.457, pgs. 706-710 (2009) (hereinafter “Kim”), the contents of each ofwhich are incorporated by reference herein. Chemical exfoliation mayalso be used to form the graphene sheets. These techniques are known tothose of skill in the art and thus are not described further herein. Theas-prepared graphene sheets typically have a sheet resistance of fromabout 250 ohms per square (ohm/sq) to about 4,000 ohm/sq, depending onthe fabrication process.

Once the graphene sheets are formed, the sheets are deposited onto asubstrate using conventional lift-off techniques. In general, the sheetsare deposited one on top of another to form the film. Thus, by way ofexample only, the graphene film can comprise a stack of multiplegraphene sheets (also called layers). The term “substrate” is used togenerally refer to any suitable substrate on which one would want todeposit a graphene film. By way of example only, the substrate can be anobject to be shielded or may be a flexible film, which may optionally betransparent. The flexible film may then be applied to the object to beshielded.

In one embodiment shown in FIG. 1, the electromagnetic shield structure10 for shielding an object 12 from electromagnetic radiation includesone or more graphene sheets 14 ¹, 14 ², . . . 14 ^(n) are transferred tothe object to be shielded. Each individual graphene sheet is doped witha dopant 15 to enhance shielding effectiveness and transparency in thevisible range. Optionally, the one or more graphene sheets aretransferred to a flexible substrate 16. In one embodiment, the flexiblesubstrate is transparent to radiation in the visible wavelength range.The number of graphene sheets utilized will vary depending on theintended application.

In another embodiment shown in FIG. 2, the electromagnetic shieldstructure 20 for shielding an object 22 from electromagnetic radiationincludes one or more graphene sheets 24 ¹, 24 ², . . . 24 ^(n) aretransferred to the object to be shielded. Doping is performed on thetransferred sheets with a dopant 25 after all of the graphene sheetshave been transferred, i.e., doping is performed on the stack.Optionally, the one or more graphene sheets are transferred to aflexible substrate 6. In one embodiment, the flexible substrate istransparent to radiation in the visible wavelength range. The number ofgraphene sheets utilized will vary depending on the intendedapplication.

As discussed above, the graphene film is doped. As used herein, the termdoped refers to an amount of dopant used to effect a dopingconcentration (n) in the graphene sheet that is reflective. By way ofexample, the dopant concentration (n) is highly doped to effectreflection and is greater than 1e10¹³ cm⁻². In other embodiments, thedopant concentration is effective to absorb the electromagneticradiation. By way of example, the dopant concentration (n) is moderatelydoped at 1e10¹³ cm⁻²>n>1e10¹² cm⁻². In other embodiments, the dopantconcentration (n) is low doped at 1e10¹² cm⁻²>n>0 cm⁻².

The dopants may be applied as a solution and/or as a vapor. By way ofexample, the graphene sheets are added to a solution of the dopant attemperatures of about room temperature to about 120° C. with agitationfor about an hour to several days. At the end of this process, thegraphene sheets are now highly doped. The residual doping agents areremoved via separation technologies (filtration wash, centrifugation,cross-flow filtration).

Examples of suitable dopants for increasing shielding effectivenessinclude oxidizing dopant such as, without limitation, hydrobromic acid,hydroiodic acid, nitric acid, sulfuric acid, oleum, hydrochloric acid,citric acid, oxalic acid, or metal salts, examples of which include, butnot limited to, gold chloride, silver nitrate, and the like. Exposingthe graphene film to the dopant solution and/or vapor shifts thegraphene Fermi level further away from the Dirac point, leading to alarge increase in the conductivity and reduction of the sheet resistancewithout interrupting the conjugated network of the graphene sheet.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

1. A method for shielding an object from electromagnetic radiation atfrequencies greater than a megahertz emitted from an electromagneticsource, comprising: encapsulating and/or enclosing the object with oneor more graphene sheets; and doping the one or more graphene sheets,wherein at least one or more of the graphene sheets are doped with adopant.
 2. The method of claim 1, wherein the dopant concentration (n)is greater than 1e10¹³ cm⁻².
 3. The method of claim 1, wherein thedopant concentration (n) is 1e10¹³ cm⁻²>n>1e10¹² cm⁻²
 4. The method ofclaim 1, wherein the dopant concentration (n) is 1e10¹² cm⁻²>n>0 cm⁻².5. The method of claim 1, wherein providing one or more graphene sheetscomprises transferring a first graphene sheet to the object; doping thefirst graphene sheet to form a doped graphene sheet; transferring atleast one additional graphene sheet to the first doped graphene sheet;and doping the at least one additional graphene sheet; wherein theprocess is repeated until a desired thickness is obtained.
 6. The methodof claim 2, where the one or more doped graphene sheets are transparentto electromagnetic radiation within the visible spectrum.
 7. (canceled)8. The method of claim 1, further comprising transferring the firstgraphene sheet to a flexible substrate, wherein the flexible substrateis disposed on or about the object.
 9. The method of claim 1, whereinthe object comprises curvilinear surfaces.
 10. The method of claim 1,wherein the dopant is selected from the group consisting of inorganicacids and metal salts.
 11. The method of claim 10, wherein the metalsalt is gold chloride.
 12. The method of claim 10, wherein the inorganicacids are selected from the group consisting of hydrobromic acid,hydroiodic acid, nitric acid, sulfuric acid, oleum, hydrochloric acid,citric acid, and oxalic acid.
 13. The method of claim 1, wherein the oneor more graphene sheets are formed by chemical vapor deposition.
 14. Amethod for shielding an object from electromagnetic radiation atfrequencies greater than a megahertz emitted from an electromagneticsource, comprising: encapsulating and/or enclosing the object with oneor more graphene sheets, wherein at least one or more of the graphenesheets are doped with a dopant in amount effective to reflect theelectromagnetic radiation.
 15. The method of claim 14, wherein providingone or more graphene sheets comprises transferring a first graphenesheet to the object; doping the first graphene sheet to form the dopedgraphene sheet; transferring at least one additional graphene sheet tothe first doped graphene sheet; and doping the at least one additionalgraphene sheet; wherein the process is repeated until a desiredthickness is obtained.
 16. The method of claim 14, wherein the dopantconcentration (n) is greater than 1e10¹³ cm⁻².
 17. The method of claim14, wherein the dopant concentration (n) is 1e10¹³ cm⁻²>n>1e10¹² cm⁻²18. The method of claim 14, wherein the dopant concentration (n) is1e10¹² cm⁻²>n>0 cm⁻².
 19. The method of claim 14, further comprisingtransferring the first graphene sheet to a flexible substrate, whereinthe flexible substrate is disposed on or about the object.
 20. Themethod of claim 14, wherein the dopant is selected from the groupconsisting of inorganic acids and metal salts.
 21. The method of claim20, wherein the metal salt is gold chloride.
 22. The method of claim 20,wherein the inorganic acids are selected from the group consisting ofhydrobromic acid, hydroiodic acid, nitric acid, sulfuric acid, oleum,hydrochloric acid, citric acid, and oxalic acid.